The present invention relates to radar displays, and more particularly, to methods and apparatus for controlling and time-sharing the creation of display data within a radar system.
A radar display as wherein the present invention is employed is shown in FIG. 1 and generally indicated as 10. Radar display 10 comprises a cabinet 12 having a large cathode ray tube (CRT) 14 disposed with its face 16 positioned for operator viewing. Cabinet 12 is further provided with an input panel 18 having switches 20, knobs 22 and other input devices, for providing range and position data etc., such as track ball 24 internally connected to be sensed by control apparatus (not shown) all of which are well known and form no part of the present invention. The control apparatus includes computing logic and electronic circuits responsive to radar returns whereby a plurality of display indicia are created on the face 16 of CRT 14. The indicia created on the face 16 of CRT 14 fall into two groups--prestored or "synthetic" data and real-time data. The synthetic data is provided by the computer from many sources and includes items such as map portions 26, land masses 28, tracking data 30, and various alpha-numeric symbols 32 of meaning to an operator. The real-time data comprises the indication of a radar beam and its returns symbolized by a rotating beam 33 rotating as indicated by the arrow 34 about an origin 36. While the rotating radar beam display 33 appears to revolve about the origin 36, generally, it is created by use of Cartesian coordinates employing an X-Y beam deflection system with the CRT 14 itself. That is, the beam is "painted" with respect to an X-Y coordinate system as indicated by the dotted lines 38 and 40 representing the X-axis and Y-axis respectively.
The creation of the two types of display (synthetic and real-time) must be on a time-share basis. That is, the logic creating the display signals to the CRT 14 must work part of the time in creating the synthetic data and part of the time to create the radar beam 33. In the past, it has been the usual practice to create the synthetic data on a priority basis. That is, at a fixed repetitive rate, the logic first paints the synthetic data on the face 16 of CRT 14 and, thereafter, uses the remaining time until time to once again paint the synthetic data to paint the radar beam 33. Until recently, this did not create a severe problem as the amount of synthetic data was minimal. In recent developments of both civilian and military air traffic control as well as air defense systems, the increase in the amount of synthetic data has been significant such that the amount of time remaining to paint the radar beam is insufficient to assure that the beam is not partially created or missing radar returns.
Recognizing the necessity to display the real-time data which is missed if the beam is not present when the data occurs, many systems have attempted to reverse the priority. That is, the beam 33 is created first and the time remaining is employed in painting the synthetic data. Unfortunately, because of the vast amounts of synthetic data which must be refreshed, the time occupied in painting the entire quantity of synthetic data one time is such that the repetition or refresh rate falls below that required to prevent flicker. That is, between the time when a symbol such as tracking data 30 is painted one time and the time when it is repainted once again, the decay rate of the phosphors on the face 16 of CRT 14 which cause the viewable display is such as to be recognizable by an operator and appears as an objectionable flicker of the display on the face 16 of CRT 14.
Various techniques have been employed to surmount this problem. For example, "cycle stealing" is often employed. That is, some of the entire beams 33 are omitted and the time that would have been employed in creating the stolen beams is dedicated to painting the synthetic data. One form of cycle stealing is to steal a fixed beam position as, for example, stealing every tenth beam. Such cycle stealing is predicated on the proposition that every target "hit" by the radar is hit by a plurality of successive beams due to the beam width of the actual radar beam. Thus, if a single beam is "stolen", the target will be detected by at least one beam on either side of the "stolen" beam.
Another form of cycle stealing is "random" stealing. This form is predicated on the proposition that if portions of beams are randomly stolen in a total quantity as necessary to gain the required time, nothing will be taken from the same location so often as to cause lost data. For a low quantity of stolen segments of beams this may have some validity. But, as a number of beam segments necessary to provide the time becomes larger, the chance that the "random" beam segments will be bunched and repeat locations in successive steals becomes larger and the opportunity for lost data becomes greater.
Such techniques have been successful to some degree, but, at some point, the trade-off between the amount of synthetic data vis-a-vis the possibility of lost real-time data becomes such that the flicker problem and lost data problem cannot be solved simultaneously by presently known techniques. That is, the chance that data will be lost becomes too large to be tolerated and another scheme must be employed.
Moreover, the internal timing or "clock" techniques employed in prior art radar systems are such as to make the calculations employed in implemanting such a scheme cumbersome and time consuming in and of themselves.
Wherefore, it is the object of the present invention to provide a method and associated apparatus performing the method for operating a radar display which assures detection of critical real-time data while, at the same time, preventing undesirable flicker in the creation of synthetic data.
It is an additional object of the present invention to provide a method and associated apparatus for operating a radar display which incorporates a technique which makes any compensating calculations employed therein straightforward and simple thereby consuming a minimum of time in and of themselves.